Method and apparatus for selectively combining signals to distinguish correlative data from non-correlative data



B. D. L EE May 23, 1967 METHOD AND APPARAT SIGNALS TO DISTINGUISHCORRELATIVE DATA FROM NON-CORRELATIVE DATA 5 Sheets-Sheet l Filed DeC.22, 1964 B. D. LEE 3,321,740

FOR SELECTIVELY COMBINING May 23, i967 5 Sheets-Sheet 2 Filed Dec. 22,1964 B. D. LEE

May 23, 1967 METHOD AND APPARATUS FOR SELECTIVELY COMBINI SIGNALS TODISTINGUISH CORRELATIVE DATA FROM NON-CORRELATIVE DATA 5 Sheets-Sheet 5Filed Dec. 22, 1964 B. D. LEE 3,321,740 METHOD AND APPARATUS FORSELECTIVELY COMBINING May 23, 1967 SIGNALS TO DISTINGUISH CORRELATIVEDATA FROM NON-CORRELATIVE DATA 5 Sheetsheet L;

Filed DeC. 22, 1964 v v Iv v ,If 1: Kiev f E@ I May 23, 1967 a. D. LEE3,321,740

METHOD AND APPARATUS FOR SELECTIVELY COMBINING SIGNALS TO DISTINGUISHCORRELATIVE DATA FROM NONCORRELATIVE DATA Filed DeC. 22, 1964 300Millseconds 300 milliseconds zoo IOO

5 Sheets-Sheet 5 l BIETHD AND APPARATUS FOR SELECTIVELY CIWBHNNG SIGNALST0 DSTINGUISH COR- LTt/ DATA FROM NON-CRRELATIVE T Burton D. Lee,Houston, Tex., assigner to Texaco Inc., New York, N.Y., a corporation ofDelaware Filed Dec. 22, 1964, Ser. No. 420,378 22 Claims. (Cl. S40-15.5)

This invention relates to the processing of electrical signals and, moreparticularly, to an improved method and apparatus for combining electricsignals in a seismic exploration system in such manner as to emphasizecorrelative information in the respective signals while suppressingnon-correlative information therein.

Seismic processing is generally carried out by introducing seismicsignals or pressure waves into the earth at a location usually referredto as a shot point by, for example, detonating an explosive charge.Geophones, eg., velocity or pressure sensitive detectors, usuallyarranged in an array at the surface of the earth, are utilized toconvert the seismic signals into correspondingly varying electricsignals. The geophones will respond to any seismic-waves or motiondetected, however, the seismic signals'of interest in a reflectionseismic system are those which are reliected from an interface orboundary between different layers of interest within the earth. Thus, itwill be appreciated, that the electric signals of interest are veryoften obscured by interferring electrical signals caused by disturbancessuch as noise both from Within the earth itself as well as surfaceconditions aifecting the geophone directly. Accordingly, Varioustechniques have been devised for accentuating the electric signals ofinterest representing reections and suppressing undesired interferencesignals.

A known technique for enhancing the reection signals is to algebraicallymix the electric signals derived from two or more seismic records orgeophones in the array usually with the result that the electricreflection signals which are repeated in each geophone output will bestrengthened and the non-corresponding electric interference signalsWill be suppressed. However, one disadvantage of this technique is thatsignal redundancy is produced. In other words, algebraic mixing appliedto the electric signals produced by geophones in the array cause theelectric signal representing the desired reection information to beimposed on the signal which is the result of mixing with other electricsignals from other geophones in the array in which the reflectioninformation signal does not appear or has not been detected. Algebraicmixing of signals also causes what is known as run-out of the reiiectionsignals on the resulting mixed signal when the electric signals beingmixed have the rellection signals thereon staggered time wise withrespect to one another. Run-out is that phenomenon wherein thereflection signals are reproduced for a number of cycles beyond theiractual occurrence. A better understanding of redundancy and run-out maybe obtained from the detailed explanation of the invention whichfollows.

A novel technique which can be applied to seismic signals or records foremphasizing correlative information While suppressing non-correlativeinformation is called like-sign mixing. The technique comprises mixingcorresponding portions of the electric signals from the geophones ortraces only if the corresponding signal portions thereof are all of thesame polarity. A zero output results from the mixing if thecorresponding portions of the electric signals are not of the samepolarity, or if any one is zero. Although the like-sign mixing techniqueper se, has the disadvantage, in certain situations, of suppressingreflection signals as well as the interference States arent O 3,321,74Patented May 23, 1967 signals, it has `been found in accordance with thepresent invention that like-sign mixing may be employed as an element ofa novel technique which does not suffer these disadvantages.

The invention provides a method as well as apparatus for accentuatingthe correlative data in a seismic retiection system and suppressing thenon-correlative signals by algebraically mixing electric signalsobtained from an array of geophones or a record of said geophone signalsfollowed by like-sign mixing of the electric signals resulting from thealgebraic mixing.

Accordingly, it is the main object of the present invention to provide amethod and apparatus for combining electric signals in such manner as toemphasize correlative information in the respective signals whilesuppressing non-correlative information therein.

It is another object of the present invention to provide a method andapparatus for mixing of electric signals without producing redundancy orrun-out of information signals.

Another object of the invention is to provide apparatus for performingan algebraic mix followed by a like-sign mix on electric signals withoutincurring the disadvantages of either.

Brieliy, the invention comprises a method of processing a plurality ofsignals in which correlative data is distinguished from non-correlativedata in the respective signals. The method includes the steps ofisolating the signals into at least two groups and algebraically mixingthe signals within each group to produce an algebraically mixed signalfrom each group. The method also includes the steps of comparing thepolarities of corresponding portions of the algebraically mixed signalfrom each of said groups and further algebraically mixing thealgebraically mixed signal from each of said groups. A portion of saidfurther algebraically mixed signal which includes the correspondingportions -of the algebrically mixed signal is passed to an output whenan agreement of polarities of the corresponding portions of thealgebraically mixed signal from each of said groups is obtained.Broadly, the invention can be described as an algebraic mixing functionfollowed by a like-sign mixing function.

The invention also includes a system for processing a plurality ofelectric signals wherein an voutput is produced in which correlativedata. is enhanced and noncorrelative data is suppressed. The systemcomprises means for isolating the electric signals into at least twogroups with means for algebracially combining the electric signals ineach group. Means are included for providing an output polarityagreement signal when the instantaneous polarities of correspondingportions of the algebraically combined signal from each of the groups isalike. Means are also provided for further algebraically combining thesignals from said groups and for applying a portion of the furtheralgebraically combined signal to the output of the system in response tothe output polarity agreement signal. The portion of the furtheralgebracially combined signal includes corresponding like polarityportions of the algebraically combined signals from said groups.

The foregoing and other objects and benefits of the invention aredescribed ybelow in greater detail and are illustrated in the drawings,in which:

FIG. 1 is a partially schematic block diagram of the system forprocessing the seismic information.

FIG. 2 is a detailed circuit diagram of the positive and negativecoincidence circuits, the or circuit, switch drive .and bi-polar switchof the system depicted in FIG. 1.

FIG. 3 is a circuit diagram of the amplifier and clipper shown in blockform on FIG. 1.

FIG. 4 is a block diagram of a digital embodiment for processing theseismic information.

FIG. 5 shows idealized record traces of seismic signals containingrefiection information which are utilized in explaining the invention.

FIG. 6 depicts record traces obtained following the application ofalgebraic mixing to the electric signals of the record traces depictedin FIG. 5.

FIG. 7 shows the record traces obtained after the application oflike-sign mixing to the electric signals of the record traces depictedin FIG. 6.

The method of the present invention is applicable to the informationderived from t-he geophone directly. That is, it is applicable to directfield operation. However, the information obtained directly in the fieldcontains serious factors which have in some situations, a destructiveeffect on the information signals. More particularly, there is aphase-time difference between the seismic signals received fromdifferent geophones due to weathering and elevation variations,requiring the corrections known in the art as static corrections, anddue to the spread geometry of the geophones, requiring the correctionsknown as dynamic corrections. If the phase-time difference is too great,there is a possibility that the information might be sufficiently out ofphase such that the algebraic mixing would produce cancellation ratherthan accentuation. Accordingly, the method is applicable to bestadvantage where the seismic signals utilized therein are reproduced froma recording wherein the phase-time difference has been compensated for.An example of means for correcting for phase-time shift may be found inU.S. Patent 2,638,402.

Referring to FIG. l, there is shown a system for processing the Seismicdata which is introduced preferably from a 4rnultitrace recorder notshown, wherein a phasetime correction has been introduced. In thisembodiment, seven traces or channels are reproduced marked 1 through 7.Each of the seven channels is connected to the respective isolationamplifier or amplifiers containing the same number as the channel. Ascan be seen there are 15 isolation amplifiers, and it will be noted thatthey are connected in groups of three. The first group being connectedto channels l, 2, 3, and the second group to channels 2, 3, 4, etc. Theisolation amplifiers themselves may be any suitable amplifier capable ofproviding the required isolation many of which are in usage today. Theoutputs from the three isolation amplifiers in each group arealgebraically mixed. This is accomplished by connecting the outputs ofthe isolation amplifiers in a group together after passing throughproportioning resistors 11, 12 and 13. The algebraically mixed outputvoltage is obtained across summing resistor 22, which is connectedbetween each of the combining points A, B, C, D and E of the groups ofthree outputs from the isolation amplifiers and ground. In a preferredembodiment the values of the proportioning resistors are chosen suchthat the resultant algebraic mix is a uniformly weighted combination.The algebraic mixing which has been performed in this particular case,is `called three trace algebraic mix. It will be appreciated that themix is not limited to t-hree traces but can be utilized with anypredetermined number of traces. Also the traces in a group do notnecessarily have to be adjacent traces. For example, one group -mightcontain traces 1, 3 and 5 and another 2, 4 and 6. This technique ofskipping traces or having groups made up of non-adjacent traces is knownin the art as skip-mixing. As can be seen, combining points A, B, C andD are connected to amplifier 33 via leads 35, 36, 37 and 38,respectively. Each of the leads 36 through 38 contains an isolationresistor 41, 42, 43 and 44, respectively. Likewise, combining points B,C, D, and E are connected to amplifier 34 via leads 55, 56, 57 and 58containing an isolation resistor 61, 62, 63 and 64, respectively. Thefunction of the proportioning resistors 11, 12, 13 will `be bestunderstood by considering the composition of the signal as it enters theD.C. amplifier 33 or 34. Assuming all the proportioning resistors 11,12, 13 are equal and isolation resistors All through 44. are, amongthemselves, equal and summing resistors 22 are equal, the inputs to theoperational amplifier 33 can be designated as (l-l-2-i-3)-|(2-{3-|-4)-l-(3-i-4-l-5) +(4-l-5-l-6). Collecting like terms we have:1(l)-l-2(2)l3(3)-}3(4)+2(5)-l-l(6) where numbers in the parenthesis areordinal numbers and represent amplitudes of original input traces. Itcan be seen that this gives an unequally weighted combination withtraces three and four being dominant. This condition is called a taperedmix and may or may not be desired. Adjustment of the values of theproportioning resistors 11, 12, 13, etc. provides control of theweighting factors and may, for instance, be so chosen that the resultantis uniformly weighted. Resistors i1-44 and 61-64 are usually of theorder of 5()` to 100 times the value of the summing resistors 22 atpoints A through E. Resistors 41 to 44 and 61 to 64 are isolationresistors. It can be seen, for instance, that signals from combiningpoint A on lead 3S must be isolated from the signals which are fed toisolation amplifier 34 from combining points B, C, D and E since A isno-t included in this group. It will be noted that the outputs of thealgebraic mixing are grouped in groups of four for the like-sign mixing.The leads 35, 36 and 37 and 38 forming one group and leads 55, 56, 57and 58 forming another group. The grouping of the outputs of thealgebraic mixing for the like-sign mixing operation is not limited togroups of four but may contain any predetermined number of outputs ineach group. However, it has been found that more reliable results areobtained by grouping a greater number of outputs from the algebraicmixing for the like-sign mixing than the number of input traces in thegroups upon which the lalgebraic mixing is performed. The aforementionedgroups of the outputs of the algebraic mixing are algebraically mixed orcombined at point H and then amplified. The amplification is necessaryto raise the level of the combined signal to a suitable level foroperation of the bipolar switch 123. The amplification is provided bywell known DC. operational amplifiers which form no part of thisinvention per se and, accordingly, no further explanation thereof isbelieved to be necessary herein.

The combining points A, B, C, D and E are connected to amplifier-clipperelements 72, 73, 74, and 76, respectively, via leads 81, 82, 83, 84 and85. The amplifier-clippers 72-76 amplify and shape the signal forfurther operation in the system. The details of a preferred embodimentof the amplifier-clipper is shown in FIG. 3 and will be described inconnection therewith. The outputs of the amplifier-clippers 72, 73, 74and 75 are connected to both positive and negative coincidence circuits91 and 92 via leads 93, 94, 95 and 96, respectively. Amplifier-clippers73, '74 and 75 are also connected to positive and negative coincidencecircuits 102 and 103 along with the output of amplifier-clipper 76. Inother words, positive and negative coincidence circuits 91 and 92compare the polarities of the outputs of amplifier-clippers 72 through75. Positive and negative coincidence circuits 1012 and 103 compare thepolarities of the outputs of amplifier-clippers 73 through 76. The twoundescribed portions of the system are similar and are connected to thepositive and negative coincidence circuits 91, 92 and 102, 103,respectively, and accordingly, only the portion connected to thepositive and negative coincidence circuits 91 and 92 will be described.

Postive coincidence circuit 91 compares the outputs ofamplifier-clippers 72, 73, 74 and '75 and produces an output on the lead112 if the amplifier-clipper outputs are all positive. Similarlynegative coincidence circuit 92 compares the outputs ofamplifier-clippers 72, 73, 74, 75 and produces an output on the lead 113if the amplifier-clipper outputs are all negative. Leads 112 and 113 areboth connected to the or circuit 114. The or circuit as its name impliesproduces an output when either one or the other of the inputs thereto isenergized. Accordingly, an output would be obtained on lead 117connecting the or circuit to switch drive circuit 118. The switch drivecircuit 118 produces an output on leads 121 and 122 which are connectedto bi-polar switch 123. The bi-polar switch 123 is driven to itsconducting condition by switch drive 118 whenever there is a coincidencesignal from either positive or negative coincident circuit 91 or 92. Thebi-polar switch 1.23 is connected to the operational D.C. amplifier 33`and when in its conducting condition, allows the output from the D.C.operational amplifier to pass therethrough to the output. In essence,what has been accomplished is that a likesign mix has been provided,that is, the mixing has been provided at combining point H and thelike-sign has been determined by the associated positive and negativecoincidence circuits. The very same situation exists with respect to thepositive and negative coincidence circuits 102, 103 which are connectedto an or circuit 131 via leads 132 and 133, respectively. The or circuit131 is connected to a switch drive 134 by means of an electricalconnection 13S and the switch drive 134 is connected to a bipolar switch136. This bi-polar switch when driven to its conducting condition byswitch drive 134 passes the algebraically mixed output from operationalamplier 34 to the output. Thus, a like-sign mix is performed on thealgebraically mixed signals from combining points A, B, C, and D.Likewise, a like-sign mix is performed on the algebraically mixedsignals from combining points B, C, D and E. It should be mentioned atthis point that the algebraic mix performed at combining point H is afour trace mix, as can be seen from FIG. 1. However, any predeterminednumber of traces could have been utilized in each group to perform thelike-sign mix operation.

In summary, the system of FIG. 1 shows the electric signals from sevenchannels being algebraically mixed in groups of three. The operationperformed is known as three trace algebraic mixing. The outputs of thegroups are combined in to further groups, this time containing fourtraces. The traces in each of these groups are algebraically mixed andare connected to the output only if the polarities of correspondingportions of the traces in the group are alike. If a polarity coincidenceis not obtained, there is a zero output. This last mentioned operationencompassing the second algebraic mix and polarity coincidence is calledlike-sign mixing. The outputs therefrom contain the correlativeinformation (corresponding portions of the traces in the original threeinput groups) which represents the reflection information in a formwhich can be easily observed, for instance in a recording trace.Accordingly, the output from the bi-polar switch can be convenientlyrecorded in a recorder 130;

yIt should be noted that many more channels of input information couldbe processed in the system by the addition of further groups in whichthe first algebraic mixing could be performed as well as subsequentgroups within which like-sign mixing could be performed.

Referring to FIG. 2 there is shown the detailed circuit diagram of thepositive and negative coincidence circuits such as 91 and 92 in FiG. 1followed by an or circuit, the switch drive circuit and the bi-polarswitch. The positive and negative coincidence circuits such as 91 and 92may be of any suitable conventional type in which an output signal isobtained only when signals of the correct polarity appear simultaneouslyat each of the several inputs.

The circuit details of preferred coincidence circuits are depicted inFIG. 2. The positive coincidence circuit consists of diodes 141 each ofwhich is connected in the output lead from the precedingamplifier-clipper.

These diodes 141 are connected in the leads such that QB the anode ofeach diode is connected to a positive voltage source through resistorl124. Also, a further diode 143 is connected to the anode of diodes 141through resistor such that it provides negative clamping. The output ofthe circuit is also connected to the anode of diodes 141 throughresistor 125. The operation of the positive coincidence circuit is suchthat the diodes 141 are conducting whenever the pulses received atpoints A, B, C or D are negative with respect to the voltage at theanode of the diodes 141. Under these conditions, these diodes conductand no output signal is derived from the coincidence circuit. However,when the input signals are sufficiently positive, that is, reach avoltage value equivalent to the voltage applied to the anode of thediodes the cut-off point of the diode is reached and there is no furtherconduction. When all of the diodes are rendered non-conductingsimultaneously, the voltage from the positive voltage source will beapplied as a positive voltage output from the positive coincidencecircuit.

The negative coincidence circuit similarly consists of a number ofdiodes 142 each of which is connected in one of the output leads fromthe preceding amplier-clipper. The diodes 142 are reversed in polaritywith respect to the diodes of the positive coincidence circuit. Thevoltage source provides a negative voltage which is applied to thecathode of the diodes 142 through resistor 126. The diode 144 isconnected to the cathode of diodes 142 through resistor 127 and isreversed with respect to diode 143 of the positive coincidence circuitthus providing positive clamping. The otuput of the negative coincidencecircuit is also connected to the cathode of diodes 142 through resistor127. As would -be expected, the operation is the same except for thepolarities involved. For instance, no output is derived from thenegative coincidence circuit as long as the inputs at a, b, c, and d arepositive with respect to the voltage applied through resistor 126 to thecathode of diode 142. However, when the inputs to all the diodes 142reach a negative value equivalent to the negative volage applied to thecathode of the diodes, cut-off of the diodes takes place and there is nofurther conduction therethrough. In this case, the output of the circuitwill be a negative voltage derived from negative source-V throughresistors 126 and 127. The negative coincidence circuit is followed by aDC. inverter stage 145 to invert the polarity of the signal appliedthereto so that the pulses applied to the next circuit from both thepositive coincidence circuit and the negative coincidence circuit arepositive.

The embodiment of the or circuit depicted in FIG. 2, consists of a pairof diodes 147 connected to the outputs of each of the positivecoincidence and negative coincidence circuits. These diodes 147 are`connected so that the positive puise app-lied to either one thereofwill pass through. The pulse obtained at the output of the or circuitfrom either one of the diodes 147 is applied to switch drive circuit118.

The switch drive or trigger circcuit 118 utilized herein has aconnection 117 for applying the output of the or circuits to the grid oftube 148 of the trigger circuit. This positive pulse applied to the gridcauses the normally non-conducting tube 148 to start conduction causingcut-off of conduction therein. The drop in plate potential of tube 148is coupled to the grid of the usually conducting tube 149. Theparticular trigger circuit utiized is a mono-stable device andaccordingly, the conduction of tube 148 is cut off and the conduction ofnormally conducting tube 149 resumes automatically. The time element ortime that 148 remains conducting before the conduction reverts to itsnormal condition is determined by the RC time constant of resistor 128and capacior 129 which are connected lbetween the plate of tube 143 andthe grid of tube 149. The trigger circuit provides signals of the rightpolarity, when in its normal conducting condition, to maintain thebi-polar switch 123 in a non-conducting or olf condition. As can beseen, the conduction of tube 148 and non-conduction of tube 149 providesa negative and a positive voltage signal respectively which allows thebi-polar switch to become conducting. The voltages are obtained from theplates of the tubes 148 and 149 and are applied to the succeeding switchvia leads 151 and 152. i

The bi-polar switch depicted in FIG. 2 consists of a bridge network inwhich each Ileg ofthe bridge contains a diode 157. These diodes are allsimilarly connected in each leg of the bridge and accordingly areconducting or non-conducting depending on the potential across thebridge legs in which the respective diodes are connected.

. A positive voltage -i-E is connected to the top nodal point of theAbridge and a negative voltage -E is connected to the bottom nodalpoint. -Ordinarily these voltages in conjunction With the input wouldtend to provide conducion of the diodes in the bridge. However, switchdrive circuit 118 in its normal conducting condition, that is with tube149 conducting and tube 148 cut-off provides a negative voltage at thetop nodal point of the `bridge via lead 152 and a positive voltage atthe bottom nodal point of the bridge via lead 151. lUnder theseconditions, none of the diodes 157 of the bridge are conducting andaccordingly an input at the left nodal point will not have` a throughpath to the output at the right nodal point. However, when the switchdrive circuit 118 is triggered by a positive input at the grid of tube148 causingv reversal of the conduction of tube 148 which event resultsin reversal of conductionin tube 149, the potential fed to the upper andlower nodal points will accordingly be reversed and the diodes 157 willall be correctly polarized for conduction so that the input at the leftnodal point will ind a through path to the right nodal point and theoutput. It will be appreciated that the circuits represented by blocksin FIG. l are not necessarily limited to the particular circuitsdescribed herein and shown in detail in FIGS. 2 and 3. These circuitsare shown to exemplify certain simple circuitry which is capable ofperforming the necessary operations represented by the blocks of FIG. l.The output from the bi-polar switch is connected to a D.C. operationalamplier 166. This amplifier is utilized to bring the signal down to itsoriginal level from the amplied Ilevel to which it was raised in orderto pass through the switch 123. FIG. 3 discloses the details of acircuit capable of performing the amplifier-clipper operation performedby elements 72-76 of FIG. l. The D.C. amplifier 161 provides reversal ofthe pulses fed thereto. The output ofthe D.C. amplifier 161 is connectedto the mid-point of a voltage divider 137, the upper end of which isconnected to a positive voltage source and the lower end thereof whichis connecied to a negative voltage source. The input to the D.C.amplifier 161 is also connected to the upper end and lower end points ofthe voltage divider through diodes 162 and 163, respectively. It will benoted that the diodes 162 and 163 are reversed with respect to eachother. The diode 162 is polarized such that the anode is connected tothe input of the amplier-clipper while diode 163 is oppositelyconnected. Accordingly, the operation is such that the input to the D.C.amplifler 161 is reversed and applied to the mid-point of voltagedivider 137 which if the positive and negative potential sources are ofequal value and the resistances on either side of the mid-point are ofequal value will usually be at a zero potential. Also, the inputdepending on its polarity, for example, if the input is positive,neither of the diodes 152, 163 conduct until the voltage reaches a valueabove the cut-off potential of diode 162 at which time the diodeconducts providing clipping of the positive input voltage. Likewise,when the input is sufficiently negative to cause conduction of diode 163clipping of the negative input is likewise provided. The function of theamplifier clipper is to convert the input signals into square waveshaving constant amplitude and having a polarity depending on that of theinput signal, with the output passing C6 through zero at the zerocrossings of the input signals. These square waves simultaneously driveone of the inputs each of a positive coincidence and a negativecoincidence circuit.

Another apparatus by means of which the instant invention can bepracticed is a digital computer. In this embodiment, the input waveformsmust be converted into a digital representation. This is accomplished bysampling or quantizing the voltage levels of the waveforms at closeintervals a high speed and converting the voltage levels obtainedthereby into corresponding digital representation. It will beappreciated that the steps of the methods can be easily performed in adigital computer. For example, the digital numbers representing theseismic traces can be stored in the computer storage such as a highspeed drum storage from which they can be read at high speed when neededto perform the algebraic mix thereon. The algebraic mix operation can beeasily perf formedin the arithmetic unit since the operation entailssimple mathematical steps, that is, addition and division by a numbercorresponding to the number of digital valuesbeing added. Of course, thetransfer of the numbers to be added from storage is under the control ofa control unit `which serves as the built in program of the computer.This unit could be the usual diode matrix mechanization. This controlcan easily control the selection of the predetermined digital numbersfrom storage representing the traces to be grouped. Likewise, ther unitcan be mechanized to control the grouping for the likesign mixing.

The like-sign mixing operation requires determining whether thecorresponding portions of the traces entering the computation are oflike sign. This can be accomplished in a variety of known ways, forexample, a simple digital comparator can be -used to determine thisfunction. lf the signs of the digital numbers representing thecorresponding portions of the traces are all of like sign, a furtheralgebraic mix is performed in the arithmetic unit. The output or resultof the digital computation can be utilized in digital form. However, inour arrangement, it has been found to be more beneficial to conve-rt thedigital result into an analog form. This is done by one of the wellknown digital to analog converters. In our case, the analog formobtained is a resulting trace which has the benefits of algebraic mixingfollowed by like-sign mixing and accordingly the desired reectioninformation is accentuated and the noise and clutter is attenuated ifnot completely removed.

FIG. 4 depicts a block diagram of a digital apparatus capable ofcarrying out the method of this invention. The input seismic traces areconverted to a digital form by an analog to digital quantizer 171. Theresulting digi- .tal representation is applied to an algebraic adder 172where the digital representations of the input seismic traces arealgebraically added in groups of three, in this case. It will beappreciated that a digital computer iS usually not a real time device asis an analog computer. Accordingly, the algebraic addition takes timeand is preferably accomplished by one adder. In other words themechanization can be time shared so that programming control and storageare necessary to perform the repetitive operations. This cuts downconsiderably on the amount of equipment necessary in situations wherelarge numbers of repetitive operations are performed.

The results of the algebraic addition applied to each group of threeinputs to the algebraic adder is shown as being applied to both amultiplier 173 and a second algebraic adder 174 via leads 181 -through184 and 186 throulgh 189, respectively. It can be seen that the outputsfrom the algebraic adder 172 are separated into groups of four. In FIG.4 only the first group is shown. The second group would consist of theleads marked (b-l-C-l-d), (c-l-d-i-e), (d-l-e-l-f) and (e-i-f-l-g). Themultiplier performs the multiplication function on the inputs theretoand provides a pulse or l output when all the inputs are designated asbeing of like sign and or no output when all the inputs are notdesignated as being of like sign.

The output of the multiplier controls an and gate 191 which has as aninput thereto the results from algebraic adder 17d. The and gaterequires that the inputs thereto be energized simultaneously to producean output. Accordingly, an output is obtained from the an gate 191 onlywhen the results of the algebraic addition are present and the output ofthe multiplier is l indicating that the inputs to the multiplier were oflike sign. The output, if any, of and gate 191 consists of the resultsof the algebraic addition which, in our case is converted to a trace bya recorder apparatus connected to a digital to analog converter 192. Theabove-described apparatus has applied an algebraic mix followed by alike-sign mix to the seismic traces and accordingly the output trace ischaracterized by accentuated reection information and attenuated noise.The main advantage of the digital mechanization over the analogmechanization heretofore described is the improved accuracy that isobtainable and more important the ability to handle a lgreat many moretraces simultaneously with a very small increase in equipment incomparison to the a-dditional equipment which would be required in theanalog mechanization.

An illustrative group of traces representing signals from a seismicexploration system is depicted in FIG. 5. The twelve traces showntherein contain equal (unit) amplitude of 121/2 cycles per second sinewaves. The phase difference from trace to trace ,is 90. A pulse of 1cycle of a 50 c.p.s. sine wave is shown on the first eight tracescentered on the ZOO-millisecond time line. An identical pulse is shownon trace number 1 centered at time 240 Ins. and 20 ms. later on eachsuccessive trace through trace 8. The 50 c.p.s. pulses have amplitudesequal to one fourth the unit amplitude of the 121/2 c.p.s. waves. The 50c.p.s. pulses illustrate reflection signals obtained in the seismicexploration while the 121/2 c.p.s. signals are representative of theinterference encountered.

FIG. 6 shows the wave forms of FIG. 5 after the application thereto of afour-trace algebraic mix. It will be noted that traces No. 1, 2 and 3have been removed from the record. Trace No. 4 `represents the sum oftraces No. 1, 2, 3, and 4 divided by four (the number of traces mixedtogether). Thus, trace No. 5 is onefourth of the sum of traces No. 2, 3,4 and 5. IRecalling that the SO c.p.s. pulses do not exist beyond trace8, it is found that the pulses on FIG 6 exist as far as trace 11. Thepulses appearing on traces 9, and 11 have amplitudes that decrease to3A, 1/2 and 1/1 of the amplitude found on trace S.

The series of staggered pulses which start centered at 240 ms. on traceNo. 1, having a period of ms. and duration of 20 ms., on FIG. 5 adds upby the four trace algebraic mixing process to produce a train of fourcycles on each trace from No. 4 through No. 8, three cycles on No. 9,two cycles on trace No. 10 and 1 cycle on trace No. 11. It is clear thatthe algebraic mixing process applied to the traces of FIG. 5 haspropagated the pulses three traces beyond the point at which theyactually terminated. This is an example of the previously referred toredundancy that results from the application of algebraio mixing. Thereproduction of the staggered pulses starting at 240 ms. to more thanone cycle on each trace is what has been previously referred to as runout.

FIG. 7 shows the effect of applying a four trace likesign mix to thetraces depicted in FIG. 6. The requirement has been imposed that a pulsemust be present on four consecutive traces in order to appear in theoutput of the like-sign mix process. The question occurs at this time asto what trace of FIG. 7 corresponds to what trace of the original tracesof FIG. 5. The pulse centered at 200 ms. does not help in answering thequestion but the pulse in the staggered train which centers at 300 ms.indicates that the first trace appearing on 10 FIG. 7 is the equivalentof trace No. 4 of FIG. 5. It should be noted that FIG. 7 depicts thepulses as ending on trace No. 8, as was true in FIG. 5 and the staggeredpulse has been restored to a single cycle rather than the wave trainwhich existed in FIG. 6.

It is worthy of note that the pulses centered at 200 ms. survived theseoperations without loss of amplitude while the staggered pulses camethrough at onefourth their original amplitude. There was no overlap ofthe staggered pulses in the original wave forms of FIG. 5 nor is thereany in the wave forms of FIG. 7. It will be appreciated that anadditional step of like-sign mixing applied to the signals representedby the wave forms of FIG. 7 would cause the staggered pulses todisappear from the traces. This would require only a twotrace like signmix to bring it about.

It will be appreciated that the 50 c.p.s. pulses represented in thetraces of FIG. 5 could not be obtained by the application of a like-signmix directly but requires the application of algebraic mixing followedby the likesign mixing. Actually, the application to the traces of FIG.5 of a three trace or greater like-sign mix would produce a zero outputat all times. That such a result would be obtained can be seen from aconsideration of the result that would be obtained from the applicationof a two trace like-sign mix to the signals of FIG. 5. Keeping in mindthat the 121/2 c.p.s. waves are dominant in amplitude, being four timesas large as the 50 c.p.s. pulses, and that the phase difference betweenadjacent traces of the 121/2 c.p.s. waves results in the adjacent tracesbeing unlike in sign half of the time with the output of the processresulting in zeros half of the time. It should be noted that a non-zerooutput portion of any given trace of a record resulting from a two tracelike-sign mix would have coincidence in time with a zero portion of itsimmediate neighbor. Since zero is, by definition in the like-sign mixprocess, an unlike sign, it can be seen that one more step of likesignmixing will cause the result to collapse to zero at all times. It wouldnot matter how many times the process were repeated beyond the threetrace like-sign mix step; the output would remain zero.

It has thus been demonstrated that desired reflection information may beaccentuated and the interference signals attenuated by the successiveapplication of algebraic mixing followed by like-sign mixing and thatthe equivalent result cannot be obtained by either process alone nor bythe two processes in reverse sequence.

Obviously, many modifications and variations of the invention ashereinabove set forth, may be made without departing from the spirit andscope thereof, and therefore, only such limitations should be imposed asare indicated in the appended claims.

I claim:

1. A method of processing at least three signals in which correlativedata is distinguished from non-correlative data in the respectivesignals, comprising the steps of arranging said signals into at leasttwo groups, algebraically mixing the signals within each group andproducing an algebraically mixed signal from each group, comparing thepolarities of corresponding portions of the algebraically mixed signalfrom each of said groups, further algebraically mixing the algebraicallymixed signal from each of said groups when an agreement of polarities ofthe corresponding portions of the algebraically mixed signal from eachof said groups is obtained, and passing that portion of said furtheralgebraically mixed signal which includes said corresponding portions toan output as an output signal and providing no output signal at saidoutput when said agreement of polarities is not obtained.

2. A method of processing at least three signals in which correlativedata is distinguished from non-correlative data in the respectivesignals, comprising the steps of arranging said signals into at leastthree first groups,

algebraically mixing the signals within each group and producing analgebraically mixed signal from each group, arranging the algebraicallymixed signals from said rst groups into second groups, comparing thepolarities of corresponding portions of each of said algebraically mixedsignals in each of said second groups, further algebraically mixing thealgebraically mixed signals in each of said second groups when anagreement of the polarities of the corresponding portions of thealgebraically mixed signals of the corresponding second group isobtained, and passing that portion of said further algebraically mixedsignals which includes said corresponding portions from each of saidsecond groups to an output as an output signal and providing no outputsignal at said output when said agreement of polarities is not obtained.

3. A method according to claim 1, wherein said groups each contain apredetermined equal number of signals, the signals in each group beingshifted from the correspondingly positioned signals in adjacent groupsby a one signal displacement.

4. A method according to claim 2, wherein said second groups eachcontain a predetermined equal number of algebraically mixed signals, andcorrespondingly positioned algebraically mixed signals in adjacentgroups are shifted with respect to one another by a one signaldisplacement.

5. A method according to claim 1, comprising the further step ofweighting said signals in each of said groups so as to predeterminetheir proportionate contribution to the subsequent algebraic mix.

`6. A method according to claim 1, comprising the fgurther step ofamplifying and shaping the algebraically mixed signal from each of saidgroups in order to provide a better comparison of the polarities ofcorresponding portions thereof.

7. A method of processing at least three seismic data signals so thatcorrelative reflection data in the signals will be enhanced andnon-correlative data will be suppressed, comprising the steps ofarranging said seismic data signals into at least two groups,algebraically mixing the signals within each group and providing analgebraically mixed signal from each group, comparing the polarities ofcorresponding portions of the algebraically mixed signal from eachof-said groups, further algebraically mixing the algebraically mixedsignal from each of said groups when an agreement of polarities of thecorresponding portions of said algebraically mixed signal from each ofsaid groups is obtained, and passing that portion of said furtheralgebraically mixed signal which includes said corresponding portions toan output as an output signal and providing no output signal at saidoutput when said agreement of polarities is not obtained.

8. A method for processing at least three seismic data signals whereincorrelative reflection data in the signals is enhanced andnon-correlative data is suppressed, comprising the steps of isolatingsaid seismic data signals into at least three first groups,algebraically mixing the seismic data signals in each of said rstgroups, arranging the algebraically mixed signals from said rst groupsinto second groups, comparing the polarities of corresponding portionsof the algebraically mixed signals from said rst groups in each of saidsecond groups, further algebraically combining the algebraically mixedsignals from said first groups in each of said second groups when thepolarities of the corresponding portions are alike, and passing to anoutput as an output signal that portion of said further algebraicallycombined signal which includes the corresponding portions of thealgebraically mixed signals from said first group and providing nooutput signal at said output when said agreement of polarities is notobtained.

9. A method according to claim 7 wherein said groups each contain apredetermined equal number of seismic data signals, and the seismic datasignals in each group are shifted from the correspondingly positionedsignals in adjacent groups by a one signal displacement.

10. A method according to claim 8, wherein said second groups eachcontain a predetermined equal number of seismic data signals, ,andcorrespondingly positioned seismic data signals in adjacent groups areshifted with respect to one another by a one signal displacement.

11. A method of processing at least three signals wherein correlativedata is distinguished from non-correlative data in the respectivesignals, comprising the steps of comparing the polarities ofcorresponding portions of the plurality of signals, algebraically mixingthe plurality of signals when said corresponding portions of saidplurality of signals have like polarities, and producing an outputsignal comprising the algebraically mixed corresponding portions of saidsignals and providing no output signal when said agreement of polarities.is not obtained.

12. A method according to claim 11, comprising the further steps ofamplifying and shaping said signals before said comparison step.

13. A method for processing at least three seismic data signals whereincorrelative data is enhanced and non-correlative data is suppressed,comprising the steps of comparing corresponding portions of said seismicdata signals, algebraically combining said seismic data signals whensaid corresponding portions of said seismic data signals have a likepolarity, and producing an output signal comprising said correspondingportions of said algebraically combined seismic data signals andproviding no output signal when said agreement of polarities is notobtained.

14. A system for processing at `least three electric signals wherein anoutput is produced in which correlative data is enhanced andnon-correlative data is suppressed, comprising means for separating saidelectric signals into at least two groups, means for algebraicallycombining the electric signals in each group, means for providing apolarity agreement signal when the instantaneous polarities ofcorresponding portions of the algebraically combined signal from each ofsaid groups is alike, means for further algebraically combining thealgebraically combined signal from said groups in response to saidpolarity agreement signal, means for applying a portion of the furtheralgebraically combined signal to the output of said system, said portionof the vfurther algebraically combined signal including correspondingportions of the algebraically combined signal from each of said groupshaving like polarities.

15. A system according to claim 14, wherein said means for applying aportion of the further algebraically combined signal to the output ofsaid system comprises a switching means and a switch drive means saidswitch drive means being responsive to said polarity agreement signaland providing the bias to operate said switch means to connect saidportion of the further algebraically combined signal to said systemoutput.

16. A system for processing at least three electrical signals whereinoutputs are produced in which correlative data is enhanced andnon-correlative data is suppressed comprising means for isolating saidelectrical signals into at least three rst groups, means foralgebraically combining the electric sigals in each of said groups,means for isolating the algebraically combined electric signals fromsaid irst groups into second groups, means connected in each of saidsecond groups for producing a polarity agreement signal when theinstantaneous polarities of corresponding portions of each of saidalgebraically combined signals from -said first groups in a second groupare alike, means for providing a further algebraic combining of thealgebraically combined signals from said rst group in each of saidsecond groups when a polarity agreement signal is provided for thecorresponding group in said second groups, means for connecting aportion of the further algebraically combined signal from Ieach of saidsecond groups to a corresponding output of the system, said portion ofthe further algebraically combined signal including the correspondingportions of each yof said algebraically combined signals from said firstgroups in a second group having like instantaneous polarities.

17. A system according to claim 14, wherein a predetermined equal numberof electrical signals are isolated to form each of said groups, theelectrical signals in each group being shifted from the correspondinglypositioned electrical signals in adjacent groups by a one signaldisplacement.

1S. A system according to claim 16, wherein a predetermined equal numberof electrical signals are isolated to form each of said second groups,the electrical signals in each of said second groups being shifted fromthe electrical signals in adjacent groups by a one signal shift.

19. A system according to claim 14, wherein Weighting means are providedfor each of said electr-ical signals in each of said groups topredetermine their proportionate contribution to the subsequentalgebraic mix.

20. A system according to claim 14, further comprising amplifying andshaping means connected between said groups of electrical signals andsaid polarity agreement means to amplify and shape said alge'braicallycombined signals.

21. A system for processing at least three seismic data signals and forproducing an output in which correlative reection pulses are enhancedand non-correlative data pulses are suppressed, comprising means forisolating said seismic data signals into at least two groups, algebraiccombining means for producing a mixed seismic data signal output fromeach of said groups, polarity agreement means connected to each of saidgroups for producing a polarity agreement signal when the instantaneouspola-rities of corresponding portions of each mixed seismic data signalfrom said groups are alike, second algebraic combining means connectedto each of said groups for producing an output in which the mixedseismic data signals from each of said groups are algebraically combinedupon the reception of the polarity agreement signal from said polarityagreement means, and switching means connected to said polarityagreement means and said second algebraic combining means for connectinga portion of the output from said second algebraic combining means tothe output of said system, said portion of the output from said secondalgebraic combining means including the corresponding portions of eachmixed seismic data signal from said groups having like polarities.

22. A system for processing at least three seismic data signals andproducing an output in which correlative reiiection pulses in thesignals will be enhanced and noncorrelative pulses will be suppressed,comprising algebraic mixing means for combining the :seismic datasignals into a single seismic data 'output signal, polarity agreementmeans connected in parallel with said algebraic mixing means and towhich each of said plurality of seismic data signals is applied, saidpolarity agreement means producing an output signal when theinstantaneous polarities of corresponding portions of each of saidseismic data signals are alike, and switching means connected to saidpolarity agreement means and said algebraic mixing means for connectinga portion of the single seismic data output signal to the output of saidsystem upon receiving said output signal from said polarity agreementmeans, said portion of the single seismic data output signal includingthe corresponding portions of each of said seismic data signals havinginstantaneous like polarities.

References Cited by the Examiner UNITED STATES PATENTS 2/1962 BucyS40-15.5 9/1964 Klein et al. 340-l5.5

1. A METHOD OF PROCESSING AT LEAST THREE SIGNALS IN WHICH CORRELATIVEDATA IS DISTINGUISHED FROM NON-CORRELATIVE DATA IN THE RESPECTIVESIGNALS, COMPRISING THE STEPS OF ARRANGING SAID SIGNALS INTO AT LEASTTWO GROUPS, ALGEBRACICALLY MIXING THE SIGNALS WITHIN EACH GROUP ANDPRODUCING AN ALGEBRAICALLY MIXED SIGNAL FROM EACH GROUP, COMPARING THEPOLARITIES OF CORRESPONDING PORTIONS OF THE ALGEBRACIALLY MIXED SIGNALFROM EACH OF SAID GROUPS, FURTHER ALGEBRACIALLY MIXING THE ALGEBRACIALLYMIXED SIGNAL FROM EACH OF SAID GROUPS WHEN AN AGREEMENT OF POLARITIES OFTHE CORRESPONDING PORTIONS OF THE ALGEBRAICALLY MIXED SIGNAL FROM EACHOF SAID GROUPS IS OBTAINED, AND PASSING THAT PORTION OF SAID FURTHERALGEBRAICALLY MIXED SIGNAL WHICH INCLUDES SAID CORRESPONDING PORTIONS TOAN OUTPUT AN OUTPUT SIGNAL AND PROVIDING NO OUTPUT SIGNAL AT SAID OUTPUTWHEN SAID AGREEMENT OF POLARITIES IS NOT OBTAINED.